In recent years, X-ray CT apparatuses that perform a photon counting Computed Tomography (CT) by employing a photon-counting type detector have been developed. Unlike integral-type detectors used in conventional X-ray CT apparatuses, the photon-counting type detector outputs signals that make it possible to individually count photons derived from X-rays that have passed through an examined subject (hereinafter, a “subject”). Accordingly, by performing the photon counting CT, it is possible to reconstruct an X-ray CT image having a high Signal-per-Noise (S/N) ratio.
Further, the signals output by the photon-counting type detector can be used for measuring (discriminating) an energy level of each of the counted photons. Accordingly, by performing the photon counting CT, it is possible to image data acquired by radiating X-rays while using one type of X-ray tube voltage in such a manner that the data is divided into a plurality of energy components. For example, by performing the photon counting CT, it is possible to generate an image that makes it possible to identify one or more substances by utilizing differences in K absorption edges.
By performing the photon counting CT, it is possible to accurately measure the radiation (the X-rays) if the dose of the incident radiation is small. However, during the photon counting CT, if the dose of the incident radiation is large, a phenomenon called “pile-up” may occur where pieces of data obtained by counting the individual photons pile up. In that situation, because it is not possible to separate the individual photons from one another, miscounts occur where the count characteristics are not linear.